In the field of lead frames adapted to be used for a semiconductor device, since semiconductor chips have become highly integrated, multi-pin lead frames having a large number of leads and a fine lead pattern structure have been developed and produced. In this connection, in order to make a fine lead pattern structure on a lead frame, a relatively thin material has been used as a lead frame base and an etching process has been widely adopted to form such fine micro-patterns, since the etching process is more suitable than other processes, such as a punching process, to easily form such fine patterns.
In a conventional method for manufacturing a lead frame, however, there has been a limitation on forming a very fine lead pattern. Thus, a TAB (tape automated bonding) tape lead frame has been developed and used, on which a finer pattern could be formed than on a conventional metal lead frame.
A TAB tape can be made by forming a thin conductive film on an electrically insulative base film and etching the conductive film to form a desired conductive pattern. Using such a TAB tape, since the conductive patterns are supported on a thin flexible base film, the thickness of such conductive patterns can be reduced to about several tens of .mu.m. Thus, it becomes possible to make very fine patterns of leads which could not be attained on a conventional metal lead frame.
On the other hand, a semiconductor device is conventionally made as follows. After the inner leads of the lead frame are electrically connected to the semiconductor chip by a wire-bonding process, the lead frame is hermetically sealed with resin to obtain a product. In a case where a TAB tape is used, the lead frame is also hermetically sealed with resin to obtain a semiconductor device product.
However, as mentioned above, since a TAB tape includes a plurality of leads which are made of conductive thin film, there has been a problem that the leads are not of sufficient strength and it is difficult to handle same, such as when the product is mounted on a printed circuit board.
The above-mentioned problems concerning a TAB-tape also arise when a so-called single-layer-type TAB tape is used to make a lead frame, in which the leads are formed of a very thin conductive material with a thickness of not more than 100 .mu.m.
In addition, after the leads have been formed, it is necessary to prevent the leads from being deformed or bent. Thus, it becomes necessary that the outer leads have a certain strength to stably and accurately mount the product on a printed circuit board without any deformation of the leads.